The present invention relates to pneumatic tires and in particular to tires having a radial carcass and a reinforced crown.
The invention has as an object to improve the construction of such tires with a view to endowing them with better endurance and improved behaviour on the road.
In the conventional present day construction, the beads of tires each contain at least one inextensible beadwire surmounted by a filler strip of hard rubber generally of triangular cross section, the carcass ply or plies being folded around this combination of wire and strip.
The main function of the filler strip is to bring about a gradual transition in stiffness between the very stiff bottom part of the bead and the thin and flexible part of the side-wall. This transition may be adjusted to give a desired increase in such and such a property of the tire, such for example as the standard of ride it gives or its properties of road holding and road behaviour. This may be done by making the filler strip of greater or lesser hardness and height. However, the opportunities in this direction are limited by manufacturing requirements and the danger of detracting from other properties of the tire. Thus, hard rubber filler strips of considerable height (extending for example half way up the side-wall) are favourable to good road behaviour (ability to hold a straight line and rapid response to changes of direction) but their endurance is poor under repeated vertical flexure of the side-walls, with the result that tire failures occur as a result of breaks in the filler strips and in the carcass plies just above the edges of the rim. Filler strips of small height produce tires in which the endurance of the beads is better but whose behaviour on the road is worse. This fault can be remedied by inserting additional reinforcing members such as flippers (strips of fabric folded around the bead-wire and filler combination), reinforcing strips applied to the folded edges of the carcass, or transition rubbers which form an extension of the filler strip in the direction of the side-wall. However, these palliatives increase the number of items which have to be assembled, the cost of manufacture and the risks of bad workmanship.
As to the nature of the filler strips, it has already been proposed, as a variation on conventional mixtures containing large proportions of reinforcing filler materials, to use mixtures of rubber and intermingled fibres masticated with the rubber. A fibrous mixture of this kind makes it possible to obtain the high levels of hardness which are generally required for filler strips but they in no way overcome the drawbacks mentioned. If the filler strip is of considerable height it breaks and if it is short it does not give good road behaviour without recourse to additional reinforcing members.